Trucks such as pickup trucks and the like are often used to transport small cargo loads and/or to make deliveries and pickups. One of the difficulties of using such trucks is that of loading and unloading heavy or awkward cargo when a loading dock of proper height is not available. For example, the cargo box of a pickup truck can be three or more feet off of the ground, making it difficult to load or unload cargo to and from the cargo box.
Various attempts have been made in the past to make it easier to load and unload cargo. For example, Canadian patent application no. 2,145,259, published Sep. 23, 1995, illustrates a vehicle having a U-shaped chassis with a drop tray decking. The tray is powered by hydraulics at each corner of the decking to raise and lower the tray. However, such a system requires a sophisticated control system for activating the hydraulics to ensure that each of the hydraulic cylinders operate simultaneously.
U.S. Pat. Nos. 4,902,188 to Page and 5,288,197 to Harris show systems wherein the cargo platform of a vehicle and a trailer, respectively, is lowered by pivoting the platform rearward and downward to facilitate easier loading and unloading of cargo. These systems suffer from the fact that they require a clear space behind the vehicle or trailer to allow the cargo platform to move rearward as it is pivoted downward.
Vehicles according to U.S. Pat. Nos. 4,902,188 to Page were briefly offered for sale in the United States under the trademark KELPIE KARGO. However, such vehicles were not commercially successful.
In order for a cargo box to be fully elevating, the rear suspension of the vehicle must be removed and replaced with an independent suspension assembly mounted on each side of the vehicle. Each suspension assembly must be sized to fit within the arm of the U-shaped cargo area. There exists problems in designing a suspension assembly which is able to not only carry the cargo load but also able to carry the load over adverse conditions and at sustained speeds.
For instance, small side to side deflections as experienced in a prior art trailing link suspension assembly will result in stability problems when the vehicle is travelling at high rates of speed and when the vehicle is turning. This problem becomes particularly acute in a vehicle having an elevating cargo box since sway bars and other stabilizing bars cannot be utilized.